A device for forming pellets of thermoplastic resin is well known from the past, and, generally, such a device has an extruder, a die which is fitted to the end of this extruder, and a cutter; a resin material which has been melted and mulled is extruded from the die by the extruder, and is cut by the cutter, thus manufacturing pellets of the desired size. As methods for cutting the resin material which is extruded from the nozzle of the die, there are known the cold cut method and the hot cut method. The cold cut method is a method in which the resin material which has been extruded from the nozzle of the die is conducted to a water tank and is cooled, and is cut up after having thus been formed into the shape of strands. On the other hand, the hot cut method is a method in which the end face of a die in which a plurality of nozzles are opened is contacted with a flow of water, and the high temperature resin is cut with the cutter directly after having been extruded in the flow of water.
In granulation by the hot cut method, since the resin is cut while in the state of not yet having fully hardened, accordingly there is no powdering of the resin, which is the disadvantage of the cold cut method. Furthermore, with granulation by the hot cut method, there are the beneficial aspects that it is possible to obtain spherical granules, and so on.
FIGS. 4 to 6 are figures showing an example of a granulating die which is used in a prior art type hot cut method; FIG. 4 is a sectional view of a granulating die A which has been fitted to the end of an extruder not shown in the figures and of a portion of a chamber 12 (a cutter chamber), FIG. 5 is a sectional view thereof taken along the line I-I in FIG. 4, and FIG. 6 is an exploded view of a tubular flow conduit portion, showing the arrangement of the tubular flow conduit.
This die A for granulation has a die holder 1 which is fixed to the tip of the extruder, and a die body 5 which is fixed to the end of this die holder 1. The interior of this die holder 1, which is formed in a tubular shape, is connected to the end of the extruder, thus constituting a melted resin flow conduit 2. Furthermore, the reference symbol 3 denotes a heater for the die holder portion, while 4 denotes bolts which attach the die main body 5. Within the die main body 5 there are provided a plurality of tubular flow conduits 7 which, with connecting to the melted resin flow conduit 2, also connect to a plurality of nozzles 8 which open to a resin discharge surface 5a formed of the die main body 5, with the nozzles 8 being provided around a circle defined upon the resin discharge surface 5a. A plurality of rod shaped heaters 6 are inserted into the die main body 5. With this granulating die A, resin passes from the end of the extruder along the melted resin flow conduit 2 and the tubular flow conduits 7, and is extruded from the plurality of nozzles 8 which are provided upon the resin discharge surface 5a. With this prior art type granulating die A, as shown in FIG. 6, the nozzles 8 and the tubular flow conduits 7 are provided at equal intervals along the circle which is defined upon the resin discharge surface 5a. 
The chamber 12 which is communicated to the resin discharge surface 5a of this granulating die A houses a cutter which has a cutter rotation shaft 9, a cutter blade support member 10, and a blade for cutting 11, and is provided with a process water inlet 13 and a process water outlet 14. In this chamber 12, while the cutter is being rotationally driven within the flow of water, the resin which has been discharged from the resin discharge surface 5a is cut immediately in the flow of water, and the granules which have been obtained are carried out with the outflow of water through the process water outlet 14 from the chamber 12.
However, since in this hot cut method the resin discharge surface of the die is in contact with the flow of water, heat is taken out from the resin discharge surface into the flow of water, so that it may happen that the temperature of the interior of the die, at least locally, may drop below the melting point of the resin. As a result, there is a danger that the holes in the nozzles may clog up, so that the productivity of the device is seriously deteriorated. Moreover, even if such clogging does not occur, it may also happen that the diameters of one or more of the nozzles may become narrowed down, so that the diameter of the pellets or granules produced may become irregular, and in this case the quality of the product granules is seriously negatively affected. Even further, if a large amount of clogging takes place, along with it becoming impossible to extrude the resin from the die, the pressure upstream of the die may become abnormally high, and this may exert a negative influence upon an upstream device, such as for example an extruder or the like.
In relation to a granulating die for use in granulation by the hot cut method there has heretofore been proposed a granulating die which has a rod shaped heater in the center of each of a plurality of flow conduits within the die and a plurality of nozzles which is provided corresponding to each of these flow conduits so as to heat up the nozzles uniformly (in, for example, Japanese Unexamined Patent Application, First Publication No. H07-178726) as a technique for preventing clogging of the nozzles.
Furthermore, a granulating die has been proposed (in, for example, Japanese Unexamined Patent Application, First Publication No. H11-58374) in which, in order to prevent dropping of the temperature in the interior of the granulating die, the vicinity in which the nozzles of the granulating die open is covered over with a porous metallic material which has a low thermal conductivity.
However, even if the above described prior art techniques are utilized in order to prevent clogging of the nozzles of the granulating die, the beneficial effect of preventing nozzle clogging has been insufficient. In particular, when pelletizing the expandable resin granules for manufacturing a foamed product by adding a blowing agent to the thermoplastic resin in the hot cut method, since the extruded resin should be formed in the water of which the temperature for manufacturing usual non-expandable resin granules (normally 60° C. to 80° C.), then it is necessary to set the temperature of the process water to a lower temperature than the normally temperature, so that as a result clogging of the nozzles may occur particularly easily, and accordingly problems arise in that the productivity is low, and the diameter of the granules which are produced becomes unstable.
As a result of assiduous investigations with regard to the problem of nozzle clogging with a granulating die to be used in the hot cut method, the present inventors have realized that there is a relationship between those of the nozzles which may easily become clogged and the direction of the flow of the water, and furthermore that, in order to prevent clogging of the nozzles, it is effective to replenish the heat energy which is lost by contact with the water by flowing a heating medium within the die main body; and, based thereupon, the present inventors have conceived the present invention.
Thus, the present invention has been elaborated in the light of the above described problems, and it takes as its objective to provide a granulating die for the hot cut method, which is capable of producing granules of uniform diameter at good efficiency while preventing clogging of the nozzles.